Monday, August 13, 2012

Q&A: What is End Reflection and How Will Discharge Sound Ratings for Terminal Units Change in 2012

There is an acoustical phenomenon known as end reflection
that is regularly encountered in HVAC systems. It occurs whenever air flowing
in a duct reaches an outlet and suddenly expands to fill a room. Although it
might not be obvious to the casual observer, sound doesn’t necessarily travel
in the same direction as airflow. The greater the degree of expansion, the more
sound is reflected away from the room.

An acoustician might say, “End reflection is the acoustic
energy in an acoustic test duct that is prevented from entering the test space
by the impedance mismatch created by the termination of the acoustic test
duct”. In layman’s terms, whenever a rapid air expansion occurs, some portion
of the sound energy generated by the supply device (i.e. terminal unit, fan
system, air handler, room fan coil, etc) travels upstream back towards the
source. This is end reflection.

All terminal unit manufacturers test their products in
accordance with ASHRAE Standard 130 ‘Methods of Testing Air Terminal Units’.
This standard provides testing procedures for both radiated and discharge
sound. End reflection has been known to affect discharge sound readings for many
years, so the standard was amended in 1994 to specify that discharge ducts in
discharge sound tests must terminate flush to the inside wall of the test
chamber. This was necessary because the further a discharge duct projects into
the test chamber, the more end reflection occurs, effectively lowering the
sound levels measured within the test chamber.

Test data measured in accordance with ASHRAE Standard 130 is
used to produce catalog data in accordance with AHRI Standard 880 ‘Standard for
Performance Rating of Air Terminals’. The latest version of this standard
(880-2011) went into effect on January 1, 2012. It requires that manufacturers
calculate the end reflection loss (ERL) and add it back to the rated discharge
sound power levels of terminal unit products. A formula based on ASHRAE
Research Project RP-1314 is used to calculate the ERL for the dimensions of the
discharge duct used during the sound test. Although the calculated ERL is most
accurately applied to 1/3 octave sound data, manufacturers may apply it to
existing full octave sound data through 2014.

Here’s how the ERL is calculated:

First determine De, the equivalent duct diameter
(ft). If the discharge duct is round, simply use the duct diameter. In the more
likely situation that the discharge duct is rectangular, the equivalent duct
diameter must be calculated as:

De = SQRT [(4 x A) / (144 x π)]

where:

A = cross sectional area of duct (in2)

So for a terminal unit with a 15 in by 12 in discharge duct:

De =
SQRT [(4 x 180) / (144 x π)]
= 1.26 ft2

Now

ERL = 10 log [1 + (0.7 xCo/π x f x De)2]

where:

Co = Speed of sound in air (use 1128 fps)

f = Octave band center frequency (Hz)

De = Equivalent diameter of the duct (ft)

So the end reflection loss of a 15 in x 12 in discharge duct
is:

2nd Octave band (125 Hz) = 5 dB

3rd Octave band (250 Hz) = 2 dB

4th Octave band (500 Hz) = 1 dB

5th Octave band (1000 Hz) = 0 dB

6th Octave band (2000 Hz) = 0 dB

7th Octave band (4000 Hz) = 0 dB

Adding this to the existing discharge sound levels of a
fan-powered product would likely raise the NC level by 6 points. The smaller
the discharge duct is, the greater the correction will be. Since research has
shown that low frequency corrections tend to become overstated as duct sizes
get very small, the maximum correction is limited to 14 dB.

So what does all this mean?

It means that every manufacturer
will need to update all published terminal unit discharge sound performance
data and selection software to meet the latest standards. Discharge sound
levels will increase for all terminal units and smaller units will see the
largest increases. The effect on large units could be negligible.

Will the actual discharge be higher?

No. The product will
perform exactly as it did before, but now all of the sound energy will be
properly accounted for. It would be fair to say that under the previous
standard, discharge sound was in many cases being understated.

In order to change the certified performance listings
posted on the AHRI website, all participating manufactures were required to
resubmit all of their products to the program. It will probably be months
before all of the changes are complete and posted on their new website.
Although AHRI has agreed to publish a full page announcement in trade magazines
to explain why these changes are necessary, it has not yet been sent out for
membership approval.